1. Field of the Invention
The present invention relates to a flexible gear transmission mechanism and, more particularly, to a flexible gear transmission mechanism which reduces energy loss due to a frictional force.
2. Description of the Prior Art
A flexible gear mechanism comprising flexible gears is known, and is disclosed in U.S. Pat. No. 3,020,775 issued on Feb. 13, 1962, U.S. Pat. No. 3,285,099 issued on Nov. 15, 1966, U.S. Pat. No. 4,451,098 issued on May 29, 1984, and Japanese Patent Publication No. 47-36545.
A transmission apparatus using the flexible gear mechanism is commercially available as "Harmonic Drive" from Harmonic Drive Systems Inc. as one of the assignees of the present application. This apparatus is compact and lightweight and can obtain a high transmission ratio, and is applied to work machines such as a wheel drive apparatus, a machine tool, an industrial robot, a winch, and the like.
In the flexible gear mechanism, a wave generator has an elliptic cam, and a flexible ball-bearing like generator bearing arranged on the outer peripheral portion of the cam. Upon rotation of the elliptic cam, balls in the generator bearing are rotated to deform an outer race into a shape corresponding to the rotating position of the cam. A wavy motion is then generated along a flexspline, and shifts at the same speed as the rotating speed of the cam.
Balls must be arranged at equal intervals between inner and outer races of the generator bearing so as to transmit the motion of the cam to the outer race. For this purpose, a circular retainer having pockets for storing balls is provided between the inner and outer races of the generator bearing. Each pocket of the retainer is preferably formed to be sufficiently larger than the ball so as not to interfere with rotation of the ball.
However, although the inner and outer races of the generator bearing deform in an elliptical shape at the same speed as that of the cam, the retainer has a circular shape. For this reason, the inner peripheral surface of the retainer tends to be brought into contact with the inner race of the generator bearing at positions corresponding to the major axis positions of the cam. In particular, when the pockets of the retainer are formed to be sufficiently larger than the balls, such contact will occur over a wide range. Such contact causes energy loss by frictional torque, and the operation of the generator bearing may be interfered with by abrasion powder formed by contact.